TRANSIENT CHARACTERISTICS OF A CENTRIFUGAL PUMP AT RAPID STARTUP

Experimental and CFD studies were carried out on transient behavior of a centrifugal pump at rapid startup. Relationship between the transient characteristics and a flow field in the centrifugal pump was investigated during transient period of the centrifugal pump from experimental results. A single-stage, volute type centrifugal pump is used for the experiments. The pump is equipped with transparent impeller, casing, suction pipe and discharge pipe for PIV in the future. The test setup is a closed-loop and consists of a suction tank, a test pump, an ultrasonic flow meter and a flow control valve. Instantaneous pressure and flow rate were measured at suction and discharge ports with rotational speed during the transient period. The pump suction and delivery pressures were measured using strain gauge type pressure transducers. Unsteady flow rate was calculated from pressure difference between two pressure measurement points in straight pipe of the pump suction lines using the difference of inertia force. The three-dimensional incompressible flow calculation of the test pump is performed using ANSYS (R) CFX 17.1. The CFD domain is consisted of a pump suction pipe, a pump casing, an impeller and a pump delivery pipe. For all computations, a block structure mesh of around 1,550,000 elements has been used. The mesh is created with the mesh generator ANSYS (R) ICEM CFD Ver.17.1. Frozen Rotor Method is used for the steady state calculation, and Transient Rotor Method is used for the unsteady calculation. The standard SST model is used for turbulence modelling. Boundary conditions of the pump inlet and outlet are used the time history of total head and mass flow rate obtained from experiment result, respectively. The variation of pump operating point, torque and axial thrust force during transient period were related to the time dependent flow field, which was investigated using CFD, in the pump. As results of the present study, it was shown that the pump operating point in experiment were larger than quasi-steady one at early transient stage, and then the pump operating point reaches to quasi-steady one. CFD results indicated similar tendency to experimental results on the variation of the pump operating point. Moreover, variation of the torque and axial thrust force during transient period also indicated deviation from each quasi-steady change. From the experimental and CFD results, the deviation of pump operating point, torque and axial thrust force from the quasi-steady change during pump startup period occurs at a large flow rate acceleration. The reason is thought to be due to that the flow field at large flow rate change cannot develop compared with that at the quasi-steady change.